Ofcom has paved the way for Channel 4, Five and BSkyB to merge their advertising sales divisions after it withdrew the Independent Television Commission's rules on the joint selling of airtime.

Ofcom has paved the way for Channel 4, Five and BSkyB to merge their advertising sales divisions after it withdrew the Independent Television Commission's rules on the joint selling of airtime.

The decision, which brings to an end the final stage of the ITV merger, will enable broadcasters with more than 5% of net advertising revenue in the UK TV market to collaborate and sell their airtime in joint packages.

However, Ofcom has said that it will investigate any merger that would create a joint sales department with a market share of 25% or over. C4 currently has a 20% share, Sky has 11% while Five has 8%.

As a result, any merger involving C4 would automatically trigger an investigation and the possible imposition of restrictions.

Ofcom also stated that it would be prepared to investigate a smaller merger - such as one between Sky and Five - if the regulator thought it would be harmful to competition. Although the relevant broadcasters have discussed the possibility of merging their sales houses in recent months, there was a sober reaction to the ruling.

C4 and Five welcomed the decision but dismissed any immediate union, while James Wildman, executive sales director at Flextech sales arm IDS, said: "This is unlikely to result in a free-for-all."

Sky declined to comment.